Many industrial processes use large quantities of industrial gases, most prevalently the atmospheric gases, which are oxygen, nitrogen, and argon. Commonly, the gases are supplied with not more than 5000 parts per million of impurities or contaminants. Increasingly however, such industrial gases are being required with increasingly higher purity by users. In some new semiconductor manufacturing processes, the gases used must have very low levels of trace contaminants. For example, nitrogen and oxygen are often required with a purity of 99.9999% by volume. The manufacture, storage, transport, distribution and analysis of gases of such high purity has required the development of many new techniques.
Often a gas is manufactured at a site remote from where it is used. When large quantities are required, usually the gas is transported to the use site as a liquid in an insulated truck, stored as a liquid at the use site and vaporized as required for use. Liquified atmospheric gases boil at temperatures well below ambient, are often used as low temperature refrigerants, and hence are known as cryogens. As used herein, liquid cryogen shall mean liquified gas, and vapor cryogen shall mean vaporized liquid cryogen. Cryogens require special handling, transport and storage techniques. The transport of a cryogen in a trailer and transfer from the trailer to a storage tank particularly offer opportunities for contamination of the cryogen.
Thus before liquid cryogen is transferred from a trailer to a storage tank, verification of the purity of the trailer contents is required. In the verification of the purity of liquid nitrogen, for example, analysis is performed for oxygen, moisture, carbon dioxide and total hydrocarbon content. The analyzers used usually accept and analyze a stream of gas. Hence a representative sample stream of liquid cryogen needs to be drawn from the trailer's container, vaporized and delivered to the analyzers, unaltered and uncontaminated.
In the prior art, it has been common to connect a tube of constant diameter from a tap in the bottom of the trailer container to the analyzer, and allow a flow of liquid cryogen to be induced by the pressure difference between the container and the analyzer discharge. Vaporization of the sample stream was accomplished in the sampling tube by natural convection from ambient air, and at times assisted by other heating means.
A number of problems were commonly encountered. One problem was that choked flow would readily occur in the tube owing to the vaporization in the tube. Some of the sample stream then at times would surge or back flow in the sampling tube, thus altering the composition of the sample ultimately reaching the analyzer. Another problem was that in the length of tube where vaporization and hence two phase flow occurred a displacement of the liquid phase from its resultant vapor phase would occur, thus altering the composition of the sample ultimately reaching the analyzer. Still another problem was that low volatility constituents in the cryogen would freeze out and deposit on the walls of the sampling tube, a phenomenon known as plating. This would also alter the composition of the sample stream ultimately reaching the analyzer. Yet another problem was that the sampling process had to be carried out for a long time in order to purge the sampling tube of contaminants which had entered from the atmosphere during its nonuse. Thus the sampling process was wasteful of valuable cryogen as well as unduly long.
The constant diameter tube used in the prior art for drawing a liquid sample was particularly unsatisfactory for an application where an analysis of a liquid cryogen was quickly required, as in analyzing the content of a vessel involved in a process, such as a distillation column in an air separation plant. Such a tube had a long response time, that is, it took a long time for changes in composition in the column to be transmitted in the tube and be reflected at the analyzer.
The prior art also used at times a small, high pressure container to capture a quantity of liquid cryogen which would subsequently be released slowly as a stream, vaporized and delivered to an analyzer. This procedure suffered from the same problems already described, inasmuch as a constant diameter tube was used to purge and supply liquid cryogen from the source to the sample container. Also, it was found that frequently the sample container used was not of sufficient volume to supply a sample stream to adequately purge the discharge line and the analyzer, or group of analyzers in order to obtain an accurate analysis of high purity cryogen.